This invention relates generally to pressure measurement and more particularly to a method and apparatus which utilizes a capacitor pick-up for measuring the differential pressure across the wall of a vibrating membrane.
The requirements for sensing and measuring pressure have been met by various devices, including sealed bellows, strain gauges, and variable inductance or capacitance sensors. However, vibrating cylinder pressure transducers have been increasingly employed for many pressure measurement applications, primarily due to their high resolution in comparison to most other devices. This type of transducer is disclosed in Arvidson U.S. Pat. No. 3,021,711 which issued Feb. 20, 1962. As explained in the patent, a thin walled cylinder is placed in flexural mode vibration by an electromagnetic drive coil. Since the resonant frequency of the cylinder depends upon the pressure differential across its wall, an electromagnetic pick-up coil which senses the vibrational frequency of the cylinder provides an output signal that is indicative of the pressure differential between fluids acting against the opposite surfaces of the cylinder wall.
The primary difficulty with vibrating cylinder transducers has been the cross coupling or "cross talk" that occurs between the drive and pick-up coils. This cross talk is inherent in transducers which use electromagnetic coils for both drive and pick-up, and is highly undesirable because it raises the overall noise level, hampers the start of oscillation of the cylinder, and distorts the signal of the pick-up coil. Even though the coils are usually oriented at right angles with respect to one another in order to minimize the cross talk, it can never be completely eliminated because of the leakage flux that inevitably penetrates the loop of the pick-up coil, regardless of the coil orientation.
The above-mentioned perpendicular alignment of the coils necessarily limits the transducer to operation in even order flexural modes, e.g. the fourth, sixth, eighth, etc. Since the fourth order mode is one of the cleaner modes and has relatively good pressure resolution, transducers are usually operated therein (in the fourth order mode). The fifth order flexural mode has been found to be equally clean and also increases the pressure resolution approximately 37% over the resolution obtained in the fourth order. However, known prior art transducers employ two electromagnetic coils and are therefore not able to operate in the fifth order mode, despite its advantages in pressure resolution.
Another difficulty with existing transducers is that the vibrating cylinder does not act as a particularly good magnetic shield because of its thin wall construction. The outer protective cover of the transducer is inadvertently set into vibration as a result of the magnetic flux leakage through the cylinder wall. The vibration of the outer cover in turn affects the pick-up coil and causes significant perturbations in the output frequency, with resulting imprecision in the pressure measurement.
Temperature variations also affect the operation of vibrating cylinder transducers because the density and pressure of the fluids and the associated characteristics of the cylinder change along with the temperature. Accordingly, some existing units are provided with a temperature sensor and additional circuitry that is able to compensate for temperature changes. Even though temperature effects are thereby negated, the temperature sensors and circuitry are unsatisfactory from a practical standpoint because they are highly complex and expensive components which significantly increase the overall cost and maintenance requirements.
It is a principal object of the present invention to provide a unique method and apparatus for measuring pressure in a manner which eliminates the foregoing problems associated with pressure transducers employing electromagnetic coils for both drive and pick-up.
A more specific object of the invention is to provide an improved vibrating cylinder pressure transducer in which there is no cross coupling between the drive and pick-up mechanisms. It is an important feature of the invention that a capacitor pick-up unit is used. Accordingly, there is no loop area in the capacitor circuit which can be penetrated by the leakage flux from the drive coil.
A further object of the invention is to provide a pressure transducer that is able to operate in any desired flexural mode, including the relatively clean, high resolution fifth order mode. Unlike existing transducers which use electromagnetic pick-up, the subject invention employs a capacitor pick-up means which is not subject to any limitations as to its mode of operation.
Another object of the invention is to provide a pressure transducer in which the pick-up unit is isolated from the effects of vibration of the outer housing. Since the thin walled cylinder serves much better as an electrostatic shield than as an electromagnetic shield, magnetically induced vibration of the outer housing will not significantly affect the electric field of the capacitor.
An additional object of the invention is to provide a pressure transducer which includes heating means for maintaining the temperature of the transducer constant. Accordingly, undesirable temperature effects are eliminated without the necessity for complex circuitry such as that employed in existing units.
Yet another object of the invention is to provide a pressure transducer in which the drive coil operates highly efficiently with low energy requirements. The improved efficiency of the invention is achieved primarily by the use of a ferro-magnetic center support and the location of the drive coil at the center of the vibrating cylinder, which results in a higher Q than when the drive coil is above or below center with respect to the cylinder.
A still further object of the invention is to provide a pressure transducer of the character described which is durable, economical, reliable, and which achieves a high degree of pressure resolution.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.